In a hydraulic drive system for civil engineering and construction machines such as hydraulic excavators and cranes, a flow of a hydraulic fluid supplied from a hydraulic fluid supply source to an actuator is controlled by a valve apparatus including a flow control valve.
This type hydraulic drive system uses, as a hydraulic fluid supply source, means for controlling the supply pressure to be held higher a fixed value than the load pressure of the actuator. As disclosed in GB 2195745A, one example of such means is a pump regulator which implements a load sensing system for controlling the pump delivery rate such that the delivery pressure of a hydraulic pump is higher a fixed value than the load pressure. Because the hydraulic fluid is supplied with the load sensing system just at a flow rate required by the actuator, undesired supply of the hydraulic fluid is reduced, which is advantageous in economy. On the other hand, the load sensing system also has the shortcoming that the pump delivery pressure cannot be controlled after the intention of an operator because of its dependency on the load pressure. Therefore, when an inertial load such as a swing of hydraulic excavators is turned, the pump delivery pressure increases up to the setting pressure of a main relief valve irrespective of the amount of a flow control valve operated. This raises the problem that an acceleration of the inertial load is maximized and the operator suffers from a large shock.
A known one of valve apparatus for use in the hydraulic drive system implementing the above load sensing system is disclosed in JP, A, 61-88002. This disclosed valve apparatus comprises a flow control valve having a supply passage communicating with a hydraulic fluid supply source, a load passage communicating with an actuator, and a first meter-in variable restrictor disposed between the supply passage and the load passage and opened dependent on an operation amount thereof; a first signal passage branched from the load passage downstream of the first variable restrictor and including a restrictor and a check valve allowing a hydraulic fluid to flow toward the load passage; a tank passage communicating with a reservoir tank; a discharge passage for communicating the first signal passage with the tank passage; a second variable restrictor provided in the discharge passage and having its opening variable dependent on the operation amount of the flow control valve to produce in the first signal passage a control pressure different from load pressure; and a second signal passage for leading the control pressure in the first signal passage to the hydraulic fluid supply source, the valve apparatus being featured in further comprising a third signal passage for connecting the first signal passage to the upstream side of the first variable restrictor at a point between the check valve and the second variable restrictor, and a restrictor disposed in the third signal passage.
With that valve apparatus, the pressure upstream of the first variable restrictor is reduced by the restrictor in the third signal passage and then led to the first signal passage. Thus, the reduced pressure is led as the control pressure to the hydraulic fluid supply source to perform the load sensing control, so that the pump delivery pressure may be controlled not depending on the load pressure. Also, by adjusting respective openings of the restrictor in the first signal passage, the restrictor in the second signal passage, and the restrictor in the third signal passage into the appropriate relationship, the dependency on the load pressure can be assured to some extent in a range above the predetermined operation amount, so that the flow rate dependent on the operation amount of the flow control valve is obtained.
In the above valve apparatus, however, since the first signal passage is branched from the load passage downstream of the first variable restrictor and includes the restrictor, there occurs a flow of the hydraulic fluid passing from the first signal passage through the restrictor therein to the load passage under a normal condition that the operation amount of the flow control valve is so increased as to secure a predetermined differential pressure across the first variable restrictor. Accordingly, the control pressure which is produced in the first signal passage by reducing the pressure upstream of the first variable restrictor is lower than the pressure upstream of the first variable restrictor, e.g., the pump pressure, but higher than the pressure downstream of the first variable restrictor, i.e., the load pressure. Consequently, the differential pressure between the pressure upstream of the first variable restrictor and the control pressure in the first signal passage becomes smaller than the differential pressure across the first variable restrictor. Thus, if the differential pressure across the first variable restrictor is set to a desired value, the differential pressure between the pressure upstream of the first variable restrictor and the control pressure in the first signal passage would be smaller than the desired value.
The hydraulic fluid supply source for the load sensing system receives, as an input signal, the differential pressure between the delivery pressure of the hydraulic pump and the aforesaid control pressure to thereby control the delivery rate of the hydraulic pump such that the above differential pressure becomes equal to a preset target value. Accordingly, the smaller differential pressure between the pressure upstream of the first variable restrictor and the control pressure in the first signal passage implies that the target value must be set to a smaller one. The reduced target value leads to the problem that the control gain is also reduced and hunting is more likely to occur.
If the differential pressure across the first variable restrictor is set to a larger value, the aforesaid differential pressure as the input signal to the hydraulic fluid supply source for the load sensing system could be increased. But, the larger differential pressure across the first variable restrictor would increase the pressure loss in the first variable restrictor and would be undesirable from the standpoint of economy.
An object of the present invention is to provide a valve apparatus and a hydraulic drive system which can control the pump delivery pressure and the drive pressure of an actuator dependent on the operation amount of a flow control valve, and can increase the differential pressure as an input signal to a load sensing system, when the actuator is driven.